tools/bpftool: Fix build slowdown

Commit ba2fd56 ("tools/bpftool: Support passing BPFTOOL_VERSION to
make") changed BPFTOOL_VERSION to a recursively expanded variable,
forcing it to be recomputed on every expansion of CFLAGS and
dramatically slowing down the bpftool build. Restore BPFTOOL_VERSION as
a simply expanded variable, guarded by an ifeq().

Fixes: ba2fd56 ("tools/bpftool: Support passing BPFTOOL_VERSION to make")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

tools/bpf/bpftool/Makefile

@@ -30,7 +30,9 @@ LIBBPF = $(LIBBPF_PATH)libbpf.a
 LIBBPF_BOOTSTRAP_OUTPUT = $(BOOTSTRAP_OUTPUT)libbpf/
 LIBBPF_BOOTSTRAP = $(LIBBPF_BOOTSTRAP_OUTPUT)libbpf.a
 
-BPFTOOL_VERSION ?= $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
+ifeq ($(BPFTOOL_VERSION),)
+BPFTOOL_VERSION := $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
+endif
 
 $(LIBBPF_OUTPUT) $(BOOTSTRAP_OUTPUT) $(LIBBPF_BOOTSTRAP_OUTPUT):
 	$(QUIET_MKDIR)mkdir -p $@